Since 1924 when the first transfer line automatically clamped and machined automotive engine blocks for Morris, automation has been the means for mass production. A transfer machine utilizes a transfer mechanism upon which work pieces are indexed in between workstations performing a prescribed sequence of operations using simple and fast motions. Each station processes the work piece with predetermined repetitive operations. A typical work piece, referred to as a "part" in the remaining discussion, might be an automotive engine component. The transfer machine is thus a collection of asynchronous machining stations synchronized by the flow of parts. Several such machines are linked together in a line to provide for the complete machining of a part. A "process" defines the collection of dedicated operations that occur at each station. The machining parameters and the sequential order of fixture actuations define the process.
The typical transfer machine operates as follows. A part is clamped in position at each station, and automated machine tools engage the parts at several stations. When the tools are withdrawn at the stations, the parts are unclamped and are transferred simultaneously to the next station by a single transfer mechanism. The operation of the transfer machine at each station includes the tasks of clamping the parts, machining the parts, and unclamping the parts.
Relay networks are prevalent in the industry for defining the clamping and motion sequences. Relays are now giving way to ladder logic implemented using programmable computers referred to as programmable logic controllers (PLC) which, in modern transfer lines, provides for central control of the transfer machine. The PLC helps to realize the goal of full automation by providing centralized computer control to the continuous, synchronous cycle that is performed by the transfer machine.
Several times during a typical day, the transfer line may be halted for various reasons. Given the current state of modern transfer lines implemented with centralized computing with a PLC and the resulting documentation problems, it is often difficult to discern precisely what caused the stoppage and secondly, it is often difficult for the operator to determine whether it is safe to restart machining operations or to manually carry out some steps.
Ladder logic, which is actually a carryover from the hardwired control relay blueprints, is the most popular method for programming PLCs. Ladder logic is easily taught and is familiar to most plant electricians. The biggest drawback, however, is that the programming of the PLC with ladder logic is carried out at a very low level. In fact, when a PLC is programmed with ladder logic, it operates much as if it were many single bit processors, operating in parallel.
The resulting ladder logic programs are far too detailed and complex and provide no layering or separation of tasks. A two-inch thick program printout for this type of machine is common, and yet the overall process is not really apparent in such a listing, which can take many days of effort to master. The process engineer has to rely upon an electrical engineer to translate the sequence steps of the process into machine and mechanism instructions. Once installed, typically only the specialist electrician is able to interpret the instructions. This makes process improvements difficult to implement. Any change made to the program requires a corresponding modification to the associated diagnostic program and message display program as well so the ladder logic can properly reflect fault detection in the modified process. Unfortunately, in practice, records of changes made to such ladder logic program are rarely kept. Thus, the documentation on file may not accurately reflect the actual program, and the two-inch thick listing at the console is frequently not current, and thus confusing to use.
There have been attempts to solve the twin problems of documentation and process restart. The current programming methods include, in addition to ladder logic, the use of function charts, state logic, and error dynamic diagnostic indication (EDDI). One approach for addressing the demands of machine transfer lines utilizes centralized control with a PLC augmented by state logic or zone logic as described in U.S. Pat. No. 4,858,102 to Lovrenich, entitled, "Distributed Logic Control System And Method". This approach addresses the problem of restarting the transfer line after shutdown such that costly errors are avoided in transferring the part onto the next station without the previous operation having been performed.
In addition, function chart programming languages, wherein inputs and outputs are assigned to an I/O space and wherein program blocks are executed in a predetermined sequence, provide a sequential for programming control systems like transfer line machine stations. The use of modular blocks interpreted in a process control environment is well known and described in U.S. Pat. No. 4,216,528 to Robertson, entitled "Digital Computer Implementation Of A Logic Director Or Sequencer".
By utilizing state logic, local at each machine station, improper states are thus avoided. While this approach provides inherent diagnostics and prevents improper random states through random operation, the process is still not apparent to the user, and motion is not integrated. The EDDI approach is an attempt to make the PLC operate as if it were a sequential process. This lets the process be more readily apparent to the user and provides inherent diagnostics. However, random operation is difficult to model. Thus, problems associated with restart remain inadequately addressed. EDDI also does not incorporate integrated motion.
Given the complexity of these systems and the several workstations, distributed computing, which provides intelligence local to the station, is costly to achieve with current methods. Thus, modern transfer machine have remained under the central control of a PLC.
Accordingly, primary objects of the present invention are to make the process more apparent to the user, provide inherent diagnostics and error messages, permit the user to engage in random manual operations or cycling of each station, and integrate tooling motions into the other transfer line processes.